The present invention relates to a picture coding method, a picture coding apparatus, a picture decoding method and a picture decoding apparatus for selecting an optimum coding mode of a picture signal to be coded and the picture signal to be decoded (hereinafter, called a picture-to-be-coded signal) from a plurality of coding modes registered in advance, according to conditions such as a bit rate in the transmission of a coded bit stream, and for coding and decoding the picture-to-be-coded signal.
A background art relating to the present invention in a general picture coding apparatus and a general picture decoding apparatus and drawbacks of the background art are described.
FIG. 1 is a diagram showing a structure of a coded bit stream generally transmitted and received between a picture coding apparatus and a picture decoding apparatus.
As shown in FIG. 1, a coded bit stream is, for example, produced for each frame (or each image plane) of a time and is composed of picture-to-be-coded header information and coded data of each of blocks 1 to N in cases where one frame is, for example, divided into N blocks.
In the picture-to-be-coded header information, quantization method selection information is, for example, included in cases where a method for changing over from a quantization method to another quantization method on coding and decoding sides is adopted.
The coded data of each of the blocks 1 to N is composed of compressed picture data and overhead data, for example, composed of coding mode information, quantization step information and motion information respectively corresponding to each quantization mode.
Therefore, in cases where the number of coding mode types is small, an information amount of the coding modes in the coded data becomes small, so that an overhead information amount can be made small. However, there is a case that an optimum coding mode depending on a picture-to-be-coded signal does not exist, so that there is a drawback that the picture-to-be-coded signal cannot be effectively coded and a picture quality is lowered.
In contrast, in cases where a large number of coding modes for various picture-to-be-coded signals are registered in advance to improve a quality of a decoded picture, the amount of the coding modes existing in the coded data is increased, so that the amount of the overhead information is increased. Therefore, in cases where a size of the coded data is predetermined because of a fixed length coding or a variable length coding, the amount on a compressed picture data side becomes small, a picture quality is lowered on the contrary, so that there is a drawback that a coding efficiency is lowered. In particular, this drawback becomes remarkable in case of a low bit rate in which a small size of the coded data is only allowed.
Next, the above drawbacks are concretely described in a case that a picture coding method and a picture decoding method are specialized to a picture coding apparatus and a picture decoding apparatus corresponding to MPEG-4 (Moving Picture Experts Group Phase-4) which is currently in progress toward standardization by ISO/IEC JTC11/SC29/WG11.
In a video VM8.0 (ISO/IEC JTC1/SC29/WG11/N1796), it is proposed to encode each of macro-blocks in a Bidirectionally predictive-Video Object Plane (B-VOP) coding by selecting one of four macro-block coding modes (hereinafter, called MBTYPEs).
FIG. 2 is a diagram showing an MBTYPE table (MBTYPE-0) for the B-VOP coding proposed in the VM8.0. In this VM8.0, it is proposed to encode each macro-block by selecting one of four coding modes specified in the MBTYPE-0 to MBTYPE-4. As a selection criterion, it is recommended to use a predictive error electric power which is out of a standardization range. In FIG. 2, a symbol xe2x80x9cXxe2x80x9d indicates data (DQUANT, MVDf, MVDb or MVDB) used in each coding mode. The detail is described in 3.5.5 item of page 77.
As is described above, because the coding modes are limited to 4 in the VM8.0 proposal, there is a case that the macro-block cannot be effectively coded, so that there is a drawback that a picture quality is lowered.
Therefore, the above proposed B-VOP coding method is extremely shifted to deal with the low bit rate, the 4 types of coding modes are insufficient, so that a using flexibility of the B-VOP coding method is low. To prevent the drawback of the B-VOP coding method, it is proposed to prepare ten or more coding modes for each macro-block of the B-VOP. However, though the flexibility of the B-VOP coding, method in the coding is improved, the number of bits of code-words expressing each coding mode is undesirably increased, an adverse influence of the increase of the overhead information becomes large in the low bit rate coding, and a coding efficiency is lowered. In particular,, in the B-VOP coding which is not used for the prediction and in which it is sufficient to display a picture for a moment, it is sure that a ratio of Discrete Cosine Transformation (DCT) coefficients to the compressed picture data in the B-VOP coding becomes lowered as compared with that in an Intra-Video Object Plane (I-VOP) coding or a Predictive-Video Object Plane (P-VOP) coding, a ratio of the overhead information to the compressed picture data is heightened in the B-VOP coding to the contrary, so that a coding efficiency is undesirably lowered.
The present invention is provided to solve the above drawbacks, and an object of the present invention is to provide a picture coding method, a picture coding apparatus, a picture decoding method and a picture decoding apparatus in which an optimum coding mode is selected from a plurality of coding modes registered in advance to perform the coding and decoding of a picture signal, even though the condition of a bit rate is changed, while reducing the burden of overhead information included in the picture signal without lowering a coding efficiency or lowering a picture quality.
Also, in cases where the present invention is specialized to a picture coding apparatus and a picture decoding apparatus corresponding to the MPEG-4, another object of the present invention is to provide a picture coding method, a picture coding apparatus, a picture decoding method and a picture decoding apparatus, in which an optimum coding mode is selected from a plurality of coding modes registered in advance to perform the coding and decoding of a picture signal according to the B-VOP coding, even though the condition of a bit rate in the transmission of the picture signal is changed, while reducing the burden of overhead information included in the picture signal without lowering a coding efficiency or lowering a picture quality.
In a picture coding method, in which an input picture is coded while changing over a coding mode to another coding mode for each of prescribed blocks of the input picture, according to the present invention, a coding mode group is selected from a plurality of coding mode groups, which each are obtained by registering a coding mode selectable for each prescribed block, according to prescribed coding mode group selection information, a coded bit stream is produced by coding each of the prescribed blocks of the input picture according to the selected coding mode group, the prescribed coding mode group selection information is added to the coded bit stream, and the coded bit stream with the prescribed coding mode group selection information is output.
Therefore, even though the number of coding modes is increased, an amount of coding mode information is not increased, and there is an effect that a picture coding method, in which the coding can be effectively performed, can be obtained.
A picture coding apparatus, in which an input picture is coded while changing over a coding mode to another coding mode for each of prescribed blocks of the input picture, according to the present invention, comprises a plurality of coding mode groups, which each are obtained by registering a coding mode selectable for each prescribed block, coding mode group selecting means for selecting a coding mode group to be used in a coding processing according to prescribed coding mode group selection information, coding mode selecting means for selecting a coding mode from a plurality of coding modes registered in the selected coding mode group for each prescribed block, coding means for coding each block according to the selected coding mode and outputting a piece of coded data, and multiplexing means for multiplexing the coding mode group selection information, the coding modes and the pieces of coded data with each other and outputting those as a coded bit stream.
Therefore, even though the number of coding modes is increased, an amount of coding mode information is not increased, so that there is an effect that a picture coding apparatus, in which the coding can be effectively performed, can be obtained.
In a picture coding apparatus according to the present invention, a plurality of coding modes respectively selectable for each of macro-blocks, which each denote a unit of a picture-to-be-coded, is registered in each of the plurality of coding mode groups, and the coding mode group selecting means selects a coding mode group to be used for the coding from the plurality of coding mode groups according to the coding mode group selecting information for each of video object planes which each denote a picture indicating a condition of a video object at a time and each are composed of the macro-blocks.
Therefore, even though the number of coding modes is increased, an amount of coding mode information is not increased, so that there is an effect that a picture coding apparatus corresponding to the MPEG-4, in which the coding can be effectively performed, can be obtained.
In a picture coding apparatus according to the present invention, the coding means comprises a plurality of quantization means different from each other, and quantization selecting means for selecting one of the quantization means according to quantization method selection information, wherein a quantization is performed by using the selected quantization means in a coding, and the coding mode group selecting means uses the quantization method selection information as the coding mode group selection information.
Therefore, even though the number of coding modes is increased, an amount of coding mode information is not increased, so that the coding can be effectively performed on given conditions. Also, because the already-existing quantization method selection information is used as the coding mode group selection information, there is an effect that a picture coding apparatus, in which a more effective coding can be performed, can be obtained.
In a picture coding apparatus according to the present invention, the plurality of coding mode groups comprises a coding mode group corresponding to a low bit rate in which a plurality of coding modes corresponding to the low bit rate lower than a prescribed standard bit rate are registered, and a coding mode group corresponding to a high bit rate in which a plurality of coding modes corresponding to the high bit rate higher than the prescribed standard bit rate are registered.
Therefore, there is an effect that a picture coding apparatus, which can correspond to various bit rates widely ranging from the low bit rate to the high bit rate, can be obtained.
In a picture decoding method, in which a coded bit stream obtained by compressing and coding a picture is input and the picture is decoded for each of prescribed blocks of the picture, according to the present invention, coding mode group selection information is decoded from the coded bit stream, a coding mode group indicated according to the coding mode group selection information is selected from a plurality of coding mode groups in which a coding mode selectable in a coding of one block is registered for each block, and coded data is decoded from the coded bit stream for each block according to the selected coding mode group.
Therefore, even though an amount of the coding mode information existing in the coded bit stream is made small, the coded data can be accurately decoded, and there is an effect that a decoding method, in which the decoding can be effectively performed on given conditions, can be realized.
A picture decoding apparatus, in which a coded bit stream obtained by compressing and coding a picture is input and the picture is decoded for each of prescribed blocks of the picture, according to the present invention, comprises a plurality of coding mode groups in which a coding mode selectable in a coding of one block is registered for each block, coding mode group selection information decoding means for decoding coding mode group selection information from the coded bit stream, coding mode group selecting means for selecting a coding mode group indicated by the coding mode group selection information from the plurality of coding mode groups, coding mode decoding means for decoding a coding mode used in the coding of one block from the coded bit stream for each block by using the selected coding mode group, and decoding means for decoding coded data of each block from the coded bit stream according to the coding mode.
Therefore, even though an amount of the coding mode information existing in the coded bit stream is made small, the coded data can be accurately decoded, and there is an effect that a decoding apparatus, in which the decoding can be effectively performed on given conditions, can be realized.
In a picture decoding apparatus according to the present invention, a plurality of coding modes respectively selectable for each of macro-blocks, which each denote a unit of a picture-to-be-coded, is registered in each of the plurality of coding mode groups, and the coding mode group selecting means selects a coding mode group to be used for the decoding from the plurality of coding mode groups according to the coding mode group selecting information for each of video object planes which each denote a picture indicating a condition of a video object at a time and each are composed of the macro-blocks.
Therefore, even though an amount of the coding mode information existing in the coded bit stream is made small, the coded data can be accurately decoded, and there is an effect that a decoding apparatus corresponding to the MPEG-4, in which the decoding can be effectively performed on given conditions, can be realized.
In a picture decoding apparatus according to the present invention, the decoding means comprises a plurality of inverse quantization means different from each other, and inverse quantization selecting means for selecting one of the inverse quantization means according to quantization method selection information, wherein an inverse quantization is performed by using the selected inverse quantization means in a decoding, and the coding mode group selecting means uses the quantization method selection information as the coding mode group selection information.
Therefore, even though an amount of the coding mode information existing in the coded bit stream is made small, the coded data can be accurately decoded, and the decoding can be effectively performed on given conditions. Also, because the already-existing quantization method selection information is used as the coding mode group selection information, there is an effect that a picture decoding apparatus, in which a more effective decoding can be performed, can be obtained.
In a picture decoding apparatus according to the present invention, the plurality of coding mode groups comprises a coding mode group corresponding to a low bit rate in which a plurality of coding modes corresponding to the low bit rate lower than a prescribed standard bit rate are registered, and a coding mode group corresponding to a high bit rate in which a plurality of coding modes corresponding to the high bit rate higher than the prescribed standard bit rate are registered.
Therefore, there is an effect that a picture decoding apparatus, which can correspond to various bit rates widely ranging from the low bit rate to the high bit rate, can be obtained.